69-72 Hood dip

[StingRay]

I have a 69-72 GM truck hood that has a dip in the middle. This is pretty common but I've never seen anybody actually address a repair of this condition. On the pass side of the rib that runs front to back it actually has a dip. On the drivers it's lost it's crown but isn't concave. The rib itself is fairly flat in the middle section where I'd expect it to have a small crown. I have a few theories but am frankly scared to test them out. It's a very low crown panel and one wrong move it will be scrap.

There are the supports that run front to back either side of the center rib. They typically have foam between them and the skin. I've tried a few different shims to see what difference is made. Shims stiffen the hood but do not change the condition of the hood.

I'm thinking that over time the hood fluttering up and down in the middle at the rib (and they do flutter) could have any or all of the following conditions. Stretched material at the bottom of the rib as though a hat channel had it's lower flanges stretched. The rib could have over time sagged and pushed the front and rear of the hood out like a sagged roof truss. This would create a false stretch situation. The rib could have flattened out sideways pushing material on the hood side to side a little also creating a false stretch. While possible I don't see the area that is concave as actually being stretched itself and am leaning towards it being a false stretch because of another condition.

It's a pretty nice hood as these go and I'd like to make it perfect. The funny color of the hood in the pics is because it has been metal prepped and there is zinc phosphate on the surface.

I'm not looking to add another brace. Has anybody actually corrected the problem that typically causes this?